Barcelona, people here will tell you, is not only the most vital and stylish city in Spain, but the densest city in Europe. Though I've heard this factoid disputed by people who aren't from here, the fact remains that Barcelona is extremely dense: to wander through much of Barcelona is to walk through mile after mile of narrow streets embraced by beautiful old buildings, fronted by small shops.

But to hang out in Barcelona is also to taste a from of urban livability almost unknown in North America. The locals complain of the nuisances, pollution and cost of living (and it is both dirty and expensive)... while they sit for long hours in some of the best cafes and bars in Europe, eating some of the best food in the world, and surrounded by a city designed to make the street a second living room.

If, as I believe, building much denser cities is the lynchpin to any realistic strategy for building a bright green future, we'll need to learn the lessons Barcelona has to teach, and figure out how to make compact communities more vibrant and fulfilling places to live than the suburban alternative.

Because doing this in North America is no easy trick, I'm eagerly awaiting my review copy of Christopher B. Leinberger's The Option of Urbanism: Investing in a New American Dream, which my urban development friends say is kicking up real buzz. The basic message: we have already entered an age when the market for walkable urbanism has grown larger than the market for drivable suburbanism, and the main challenge now is building enough compact communities, well enough, fast enough.

You can hear an interview with Leinberger on Carol Coletta's awesome Smart City radio show here.

Here is one of the questions with which I'm struggling: if the unintended consequences of suburban sprawl include auto dependence, social stratification, obesity, loss of farmland and climate change, what might the unintended consequences be of a massive shift towards walkable communities? Gentrification, of course, stands out, but what else might we try to foresee?

The Option of Urbanism is part of our month long retrospective leading up to our anniversary on October 1. For the next four weeks, we'll celebrate five years of solutions-based, forward-thinking and innovative journalism by publishing the best of the Worldchanging archives.